1. Field of the Invention
This invention relates generally to perovskite-type catalysts which are useful in carbon monoxide oxidation, hydrocarbon oxidation, nitrogen oxide reduction and oxidation of trapped soot particles. In addition, this invention relates to perovskite-type materials displaying so-called giant magnetoresistance (GMR). Furthermore, this invention relates to a method of making perovskite-type catalysts and materials.
2. Description of Related Art
Perovskite compositions are nominally designated as ABO.sub.3, in which A represents a rare earth metal such as lanthanum, neodymium, cerium or the like, and B represents a transition metal such as cobalt, iron, nickel or the like. It is known in the art that perovskite-type materials are useful for the catalytic oxidation and reduction reactions associated with the control of automotive exhaust emissions. It is also known that perovskite materials (powders, single crystals and thin films) containing Mn on the B-site show giant magnetoresistance effect (GMR), such that on application of a magnetic field, the electrical resistivity of the material drops drastically due to a field-induced switching of the crystal structure. For this reason, GMR has attracted considerable attention for device applications such as magnetic recording heads.
Several techniques have been used to produce perovskite-type catalyst materials for the treatment of exhaust gases from internal combustion engines. The ability of such materials to effectively treat internal combustion exhaust gases depends on the three-way activity of the material, i.e. the capability for nitrogen oxide reduction, carbon monoxide oxidation and unsaturated and saturated hydrocarbon oxidation. The following patents describe such materials and techniques in the three-way catalysis application: U.S. Pat. Nos. 3,865,752; 3,865,923; 3,884,837; 3,897,367; 3,929,670; 4,001,371; 4,107,163; 4,126,580; 5,318,937. In addition to these patents there are numerous studies reported in the scientific literature relating to the fabrication and application of perovskite-type oxide materials in the treatment of internal combustion exhaust emissions. These references include Marcilly et al, J. Am. Ceram. Soc., 53 (1970) 56; Tseung et al J. Mater. Sci 5 (1970) 604; Libby Science, 171 (1971) 449; Voorhoeve et al Science, 177 (1972) 353; Voorhoeve et al Science, 180 (1973); Johnson et al Thermochimica Acta, 7 (1973) 303; Voorhoeve et al Mat. Res. Bull., 9 (1974) 655; Johnson et al Ceramic Bulletin, 55 (1976) 520; Voorhoeve et al Science, 195 (1977) 827; Baythoun et al J. Mat. Sci., 17 (1982) 2757; Chakraborty et al J. Mat. Res. 9 (1994) 986. Much of this literature and the patent literature frequently mention that the A-site of the perovskite compound can be occupied by any one of a number of lanthanide elements (e.g. Sakaguchi et al Electrochimica Acta 35 (1990) 65). In all these cases, the preparation of the final compound utilizes a single lanthanide, e.g. La.sub.2 O.sub.3. Meadowcroft in Nature 226 (1970) 847, refers to the possibility of using a mixed lanthanide source for the preparation of a low-cost perovskite material for use in an oxygen evolution/reduction electrode. U.S. Pat. No. 4,748,143 refers to the use of an ore containing a plurality of rare earth elements in the form of oxides for making oxidation catalysts.
In addition to the above mentioned techniques, other techniques have been developed for the production of perovskite materials containing Mn on the B-site which show giant magnetoresistance effect (GMR). Such materials are generally made in forms of powders, single crystals, and thin films. A common technique is the growth of single-crystals from a phase-pure perovskite source, see, for example, Asamitsu in Nature 373 (1995) 407. All such techniques use a phase-pure perovskite compound with a single lanthanide on the A-site, in addition to an alkaline earth dopant. An example of such phase-pure perovskite compound is La.sub.1-x Sr.sub.x Mn0.sub.3.
It is also known in the art that it is difficult and expensive to prepare individual rare earth compounds such as individual lanthanides. Thus, the cost is high for making perovskite-type materials with a single lanthanide on the A-site. Therefore, a need exists for using low-cost starting materials to manufacture inexpensive catalyst materials having three-way activity for use in conversion of exhausts from internal combustion engines, fuel cells, metal air batteries, treatment of exhaust from internal combustion engines and treatment of industrial waste gases with improved activity and thermal and chemical stability. A need also exists to manufacture bulk materials, thin films and single-crystals of materials showing GMR using inexpensive starting materials.